Cryptoicoccal capsular antigen determination by latex agglutination in the cerebrospinal fluid has been one of the most successful tests in diagnosing infectious diseases. Unfortunately, the application of this method to the detection of cryptococcal capsular antigen in serum has been less successful due to the interference of rheumatoid factor: Results of pre-treatment of serum with dithiotheitol or EDTA have been reported with various success. In this study one kit was clearly superior to the others and has now been put into routine use in our laboratory.